This invention relates generally to refrigeration and more particularly to microminiature refrigerators and modes of assembly.
Certain materials, called superconductors, have the ability to pass electric current without resistance. Since superconductivity is observed only at temperatures close to absolute zero, one of the main obstacles to extensive use of superconducting devices is the need for reliable, continuous refrigeration. Superconducting devices, such as supersensitive magnetometers, voltmeters, ammeters, voltage standards, current comparators, etc., require a cryogenic environment to operate. Traditionally this has been provided by a bath of liquid Helium. The Helium is liquified elsewhere and transported to, and transferred to the device Dewar. The labor and complexity of such an operation has severely limited the use of these devices. Many of the above superconducting devices dissipate only a few microwatts in operation while the available cryogenic systems provide a refrigeration capacity of watts, thus the devices are poorly matched to the refrigeration.
In addition, many devices such as optical microscope stages, x-ray diffraction sample holders, electron microscope cold stages, devices for cryosurgery in the brain, for ECG, MCG and EKG measurements, and low noise amplifiers, require or benefit from subambient operating temperatures.
Additionally, there are a number of high speed, high power devices such as VLSI (very large scale integration) chips and transmitters that are small, on the order of a centimeter square, and dissipate large amounts of heat, on the order of 10 to 50 watts. Traditional cooling devices, such as fans for convection cooling, are not capable of dissipating this amount of heat without significant increases in temperature above ambient.
Miniature closed cycle refrigerators such as those based on the Gifford-McMahon cycle, Vuilleumier, Stirling, etc., have been developed. These refrigerators with capacities in the range of 0.5-10 watts, are convenient and compact but, because of their moving parts, they introduce a large amount of vibration and magnetic noise which interferes with the operation of the devices. Miniature Joule-Thomson refrigeration systems have been developed which have a cooling capacity typically between 0.5-10 watts. The design configurations of these compact systems are generally helically finned tubes coiled around a mandrel, the high-pressure gas flowing inside the tubes and the low-pressure gas flowing outside the tubes. Such helically finned and coiled heat exchangers are fabricated by laborious welding or soldering of the individual components. Because of the intricacy of the device, microminiature refrigerators with milliwatt capacities until now have not been made.
What is needed for many devices is a microminiature refrigerator of approximately 1/2" to 4" in size with a cooling capacity in the milliwatt range. Also needed are microminiature refrigerator fabrication methods which avoid conventional laborious welding or soldering techniques and allow the formation of very small gas lines to operate the heat exchangers in the laminar flow regime and still have an efficient exchange of heat. The consequent absence of turbulence in the gas stream eliminates vibration and noise, both important considerations for superconducting device applications. The miniature size would allow the incorporation of an entire cryogenic system-superconducting sensor as a hybrid component in electronic circuitry. The microminiature refrigeration capacity would allow the matching of the refrigeration system to the load. The invention solves many of these problems.
Also needed are microminiature refrigerators of the same general dimensions as discussed above that can dissipate large amounts of heat, 10-50 watts, generated by certain small devices while maintaining ambient or subambient operating temperatures. And such refrigerators should be easy to manufacture and in configurations that are compatible with standard electronic packaging.
As explained in greater detail below, the microminiature refrigerator of the present invention comprises, in a unique form and scale a plurality of sealed plate-like members which form between them a cooling chamber, a heat exchanger capillary passage and fluid passages for conveying incoming high pressure gas successively through the heat exchanger the capillary section and into the cooling chamber. Return or outflow passages conduct the fluid from the cooling chamber through the heat exchanger in counterflow relation with the incoming gas and then to the exterior of the device.
Such a microminiature refrigerator requires scaling down a conventional refrigerator by a factor of about a thousand. The design perameters for a microminiature refrigerator of the same efficiency as a conventional refrigerator using turbulent flow are described in "Scaling of Miniature Cryocoolers to Microminiature Size," by W. A. Little, published in NBS Special Publication in April, 1978, which is hereby incorporated by reference.
In summary, the diameter d of the heat exchanger tubing, l the length of the exchanger and t the cooldown time are related to the capacity which is proportional to m the mass flow, in the following manner: EQU d.perspectiveto.(m).sup.0.5 EQU l.perspectiveto.(m).sup.0.6 EQU t.perspectiveto.(m).sup.0.6
A microminiature turbulent flow refrigerator with a capacity of a few milliwatts should have d=25.mu. and l a few centimeters.
As the device becomes smaller and smaller, eventually the mass flow becomes too small to allow turbulent flow of the fluid to occur. Laminar flow operation then becomes possible without loss of refrigeration efficiency.
The theoretical basis for designing microminiature refrigerators using laminar flow heat exchangers is discussed in "Design Considerations for Microminiature Refrigerators Using Laminar Flow Heat Exchangers," presented by W. A. Little at the Conference on Refrigeration for Cryogenic Sensors and Electronic Systems, Boulder, Colo., Oct. 6 and 7, 1980, which is hereby incorporated by reference.
For microminiature heat exchangers operating in the laminar flow region over the same pressure regime and having the same efficiency, the length of the exchanger (l) should be made proportional to the square of the diameter (d) of the exchanger tubing. For example, a Joule-Thomson exchanger operating with N.sub.2 at 120 atmospheres, with a capillary channel passage 5 cm long, 110 microns wide and 6 microns deep, should provide approximately 25 milliwatts cooling. Different refrigeration capacities and operating temperatures can be obtained by varying the width of the channel and/or refrigerant used. For example, ammonia can be used to provide a cooling capacity of 50 watts or more at -30.degree. C. One may thus operate under streamline conditions free of vibration and turbulence noise, an advantage, particularly for superconducting devices, which require a very low noise environment.
To increase the efficiency of the refrigerator for certain applications one form of the invention provides two capillary sections arranged in series or in parallel and passages for conducting a substantial portion of the gas directly to the outflow passage of the heat exchanger after passage through only one of the capillary sections.
In order to construct microminiature refrigerators, new fabrication techniques are needed for producing heat exchangers and expansion nozzles, a factor of 100 to 1,000 times smaller than those of conventional refrigerators.
Conventional fabrication techniques are ill-suited for microminiaturization since channels of the order to 5-500 microns must be formed accurately and the device must be sealed so as to withstand high pressure of the order of 150-3000 psi for refrigeration efficiency.
Accordingly, the major object of the invention is to provide a novel microminiature refrigerator particularly for cryogenic cooling and mode of assembly.
Another object of the invention is to provide a novel microminiature refrigerator with a cooling capacity ranging from milliwatts up to 50 watts or more.
Another object of the invention is a novel multilayer microminiature refrigerator.
Another object of the invention is to provide a novel refrigerator assembly having two or more multilayer refrigerators for cascade cooling to reach low cryogenic temperatures.
A further object of the invention is to provide a novel microminiature refrigerator composed of three or more similar plates of glass or equivalent material assembled in a stack with micron-sized fluid flow high-pressure inlet and low-pressure outlet passages connected to a cooling chamber and being arranged in separate layers at interfaces between adjacent plates.
Further to this object it is contemplated that in a lamination of three plates high-pressure inlet, heat exchange and capillary expansion passage sections are formed in series in one plate surface-bonded to an intermediate plate, one surface of which forms one side of the passages and low-pressure pressure return recess means is formed in the surface of the other plate that is bonded to said intermediate plate.
Another object of the invention is to provide a novel refrigeration assembly having a special mounting and gas passage defining holder at one end, as for mounting the refrigerator to extend cantilever fashion within an evacuated enclosure.
Further objects of invention will appear as the description proceeds in connection with the appended claims and the accompanying drawings.
In accordance with the invention, a microminiature refrigerator and method of making the same is provided. It should be noted that the microminiature refrigerator can be scaled up both in size and capacity for certain applications. The refrigerator is novel, irrespective of size; however, it is the miniaturization of the refrigerator that is difficult and therefore this is described in detail here. The turbulent or laminar flow microminiature refrigerator includes one or more plates on some of which micron-sized gas channels are formed and completed by one or more bonded plates resistant to high pressures. The microminiature refrigerator may be used for, but is not limited to, cryogenic refrigeration.